When casting metal melts, two casting rolls arranged axially parallel and cooled internally rotate in counter-rotation to one another in devices, also designated as “twin-roller casting machines”, said casting rolls delimiting between them the longitudinal sides of a casting gap. Into this casting gap, in each case, as much molten melt is cast such that what is referred to as a “melt pool” forms above the casting gap.
The melt which passes from this melt pool onto the casting rolls solidifies in each case to form a shell, which is then conveyed by the respective casting roll into the casting gap. The shells are then pressed against one another in the casting gap, such that the cast strip is formed from them and the melt enclosed between them. The strip continuously emerging in this way from the casting gap is drawn off, cooled, and conveyed onwards for further processing.
In order to provide good thermal conductivity, the casting rolls have as a rule a roll body, which at least in the area of its circumferential surfaces consists of a copper alloy. In practical operation, however, the casing surface of the casting rolls which comes in contact with the metal melt is subjected to severe mechanical and thermal loading. This applies in particular if the metal melt to be cast is a steel alloy. For this reason, casting rolls used for the casting of steel are, as a rule, provided on their casing surface with a coating, which has a greater hardness than the other material of the casting roll.
A method for the production of such a coating on a roll body of a casting roll consisting of copper is known from EP 0 801 154 B1. According to the known method, while respecting specific method steps, the circumferential surface consisting of copper of the casting roll is coated with a nickel layer by electrolytic means. In this way, the finished casting roll is coated with a wear-resistant layer on its casing surface which comes in contact with the melt, of which the hardness is perceptibly higher than that of the roll body in the uncoated state. In practical operation, the nickel layer protects the copper rolls from mechanical damage and reduces their thermal load.
Practical experience has shown that crack-free strips can only be cast with casting rolls which have adequately rough casing surfaces. Accordingly, the casing surface of casting rolls composed for example in the manner described heretofore, with a nickel layer applied which provides protection against excessive wear, is specifically shaped, usually by sand-blasting, shot-blasting, or comparable methods, in such a way that an adequate degree of roughness is achieved on it. In practice, surfaces are produced on the casting rolls of which the peak number (i.e., peak count) RPc, determined in accordance with the StahlEisen test specification SEP 1940 (3)/prEN 10049, of surfaces which are treated by shot-blasting lies in the range from 4-7 cm−1. The effort and expenditure associated with this coating and surface treatment is, however, considerable, and incurs high additional costs.
In order to reduce this expenditure and effort, and at the same time to improve the service life of the casting rolls, it has been proposed in JP 2003 191055 A that a coating 300-1000 μm thick be applied onto the casing surfaces of the roll bodies by flame spraying. With this thermal spraying method the spray medium, present in the form of wire, bar, or powder, is melted by means of a suitable heat source, which can be, for example, an oxyacetylene flame, and is then sprayed onto the casting roll surface to be coated with compressed air or another gas.
One advantage of the method of coating roll bodies of a casting roll known from JP 2003 191055 A is the fact that by means of flame spraying even especially hard metal alloys with high melting points can be applied onto the casing surface which is to be coated. It is likewise possible, by means of a suitable thermal spray process, to apply metal powder or ceramic particles optimally composed in respect of their hardness and wear resistance onto the surface which is to be coated.
The coating obtained from JP 2003 191055 has improved wear resistance with simplified manufacture. Practical investigations have revealed, however, that strips cast with casting rollers coated in this way do not fulfill the requirements imposed with regard to their surface quality and casting microstructure with the required degree of operational reliability.